What physical evidence links Ewell directly to the attack on Scout?

The knife is the evidence that directly links Ewell to the attack of Scout. Her costume (the wire in the costume) has a clean mark made by the knife. The wire in the costume likely saved her life.

Atticus fetched the remains of my costume. Mr. Tate turned it over and bent it around to get an idea of its former shape. “This thing probably saved her life,” he said. “Look.”He pointed with a long forefinger. A shiny clean line stood out on the dull wire. “Bob Ewell meant business,” Mr. Tate muttered.